This is a National Phase Application based on PCT International Application No. PCT/GB99/01005 filed Mar. 31, 1999.
Not Applicable
Not Applicable
1. Field of the Invention
This invention relates to channel allocation in radio telephone systems.
2. Description of Prior Art
When designing a radio channel plan for a radio network it is necessary to provide sufficient geographical distance between radio sites such that their coverage does not cause mutual interference when using a common frequency. At the same time it is desirable to re-use each radio frequency or channel as many times as possible within the network so as to make maximum use of scarce radio spectrum. The radio plan must take account of the worst conditions that could prevail. This means taking into account the most extreme interference conditions brought about by the geographical location of potentially interfering mobile radios.
A scheme which can identify the location of mobile radios in order to determine the likelihood of such interference can enable greater efficient use of the available radio channels. Where all interfering mobile radios are found to be distant to the area of potential re-use, the system can use the channel for a new call in the knowledge that sufficient radio separation has been achieved.
A particular practical application of the invention is that of an underground railway. In this application the radio environment is well defined since radio propagation is restricted to tunnel routes. Furthermore radio coverage may have been further localised by the use of radiating cable which may be cut into lengths served by separate radio base stations thereby creating radio coverage areas.
According to one aspect of the invention a radio telephone system comprises a plurality of mobile telephones, a plurality of base stations and positioning means operative to determine the position of mobile radios, the system having channel allocation means into which is loaded information regarding first and second conditions for determining channel allocation, the first condition being based on avoiding interference in areas served by the base stations and the second condition being based on avoiding interference in areas served by the positioning means, the channel allocation means being operative to determine if a channel is in use in an interfering zone by using the first condition arid, if the first condition is not met, determining if the channel is in use in an interfering sub-zone by using the second condition.
According to another aspect of the invention there is provided a method of allocating a channel to a mobile radio in a radio telephone system having a plurality of base stations, comprising determining if a channel is in use in an interfering zone by using a first condition based on avoiding interference in areas served by the base stations and, if the first condition is not met, determining if the channel is in use in an interfering sub-zone by using a second condition based on avoiding interference in areas served by positioning means distinct from the base stations.
In a preferred embodiment the positioning means are beacons in radio communication with the mobile radios. Where the invention is applied to a radio telephone system in a railway underground system, the beacons may be the train control beacons which are conventionally placed at intervals along the underground railway track. For an above ground application, the positioning means may be provided by beacons or by a more accurate positioning system, eg known positioning systems using satellites.
The method is of particular benefit where radio frequencies are scarce and must be re-used over short geographical distances. The method is particularly applicable to a tightly controlled radio environment such as a railway or other pre-defined vehicular route. The method enables radio frequencies to be re-used with greater regularity than would be possible by pure radio voting or RF propagation techniques. The invention relies on the use of the positioning means (physical location beacons or other geographical positioning systems) to identify precise location information of the mobile radio and thereby enable radio channels to be re-used more regularly without causing co-channel interference. Without the use of the invention it is necessary to allocate more radio frequencies in order to achieve the same grade of service.
Under normal circumstances the location of potentially interfering mobile radios can only be defined by the radio coverage limits of each base station. The use of location information from signalling beacons existing on the railway can enable the radio system to determine more precise location information within the coverage area. In this way a mobile radio which (for example), whilst using a radio channel, is about to exit a particular coverage area, will not cause radio interference to another radio using the same radio channel some distance behind it in the previous coverage area. By knowing that the two radios are sufficiently separated the channel can be re-used with safety.